1. Field of the Invention
This invention relates to a semiconductor laser device for use in laser printers, optical disks, bar code scanners and so on.
2. Related Background Art
As semiconductor laser devices, those which emit beams near 670 nm are already known. Such semiconductor laser devices have a structure in which an active layer of gallium/indium/phosphorus (GaInP) is sandwiched by a p-type and an n-type clad layer of aluminium/gallium/indium/phosphorus (AlGaInP) (refer to, e.g., "Control of Perpendicular Radiation Angle of AlGaInP Visible Semiconductor Laser", 28a-ZG-9, The 1989 Autumn Convention of The Society of Applied Physics). But in the above-described semiconductor laser devices the p-type clad layer tends to have high temperatures because AlGaInP has higher heat resistance compared with other semiconductors, such as AlGaAs, etc. To suppress the heating of the p-type clad layer it has been tried to decrease the resistivity of the p-type clad layer of AlGaInP by increasing a doping amount of the p-type clad layer, but because increases in a doping amount of the p-type clad layer adversely lowers an activation ratio of the dopant, it is impossible to decrease the resistivity of the p-type clad layer of AlGaInP. This phenomenon takes place also in the case that zinc atoms (Zn), magnesium atoms (Mg) and so on, which are major p-type dopants of the clad layer of AlGaInP, are doped. This phenomenon of drops of activation ratios of the dopants becomes more conspicuous as the Al composition ratio of the clad layer is increased. Thus, when the Al composition ratio of the clad layer is determined, a minimum value of the resistivity of the clad layer is substantially determined, and a minimum heat generation amount of the laser devices is also determined.
In lowering the Al composition ratio of the p-type clad layer for this reason so as to suppress decreases in an activation ratio of the dopant, it is impossible to efficiently confine carriers in the active layer. This is a cause for deterioration of the characteristics of the semiconductor laser devices. In short, higher Al composition ratios are preferable to efficiently confine carriers in the active layer. As seen from the above, increases in the efficiency of confining carriers, and the suppression of heat generation in the semiconductor laser devices are on opposite in terms of degrees of the Al composition ratio.
A first object of this invention is to provide a semiconductor laser device having high efficiency in confining carriers but a small heat generation amount.
To shorten oscillation wavelengths of the semiconductor laser devices, a bandgap of the active layer is widened. But this results in a problem that a difference in the bandgap between the active layer and the clad layer becomes smaller, and the confinement of carriers, especially electrons, is deteriorated, characteristics of the devices in high-temperature operations being deteriorated. To solve this problem a method of using multi-quantum barriers between the active layer and the p-type clad layer (e.g., Electronics Letters, 1986, vol. 22, p. 1008; 27 p-R-2, The 1990 Autumn Convention of The Society of Applied Physics) has been proposed.
But even by the use of this method, sufficiently high barriers have not been able to be obtained. In terms of the confinement of carriers, so far satisfactory effects have not been always achieved.
A second object of this invention is to provide a semiconductor laser device which can solve this problem and have barriers of wide bandgaps.